It has been recently found that random copolymers of ethylene and bulky cycloolefins are excellent in transparencey and have balanced heat resistance, heat property, chemical resistance, solvent resistance, dielectric characteristics and mechanical properties, and thus, exhibit excellent performances in the field of optical materials such as optical memory discs and optical fibers. On the basis of the above findings various technical proposals concerning the copolymers of ethylene and bulky cycloolefins are disclosed in U.S. Pat. No. 4,614,778, Japanese patent Laid-open Publication Nos. 61-98780, 61-115,916 and 61-120,816.
The inventors have found some problems that upon continuous copolymerization of ethylene with a bulky cycloolefin in a polymerization vessel equipped with a stirrer, although depending upon the reaction conditions, undesirable copolymers which have unduly high content of ethylene and are insoluble in a hydrocarbon solvent used in the polymerization reaction (referred to hereinafter as solvent-insoluble copolymers) are liable to be formed on the wall of the polymerization vessel in the vicinity of the gas-liquid interface in the vessel, that the solvent-insoluble copolymers formed on the wall of the vessel in the vicinity of the gas-liquid inerface invite changes in conditions of the gas-liquid interface with time and, when formed in large amounts, result in decrease of the effective area of the gas-liquid interface, that on that acount the copolymerization of ethylene and the cycloolefin does not proceed sufficiently, and that the solvent-insoluble copolymers formed on the wall of the vessel fall off in the liquid phase in the vessel, withdrawn through a pick up line together with the desired copolymer produced, and trapped by a filtering device installed in the pick up line to clogg the filtering device and occasionally the pick up line in itself, thereby hindering a continuous and stable running of a series of apparatus for the production of the ethylene-cycloolefin random copolymers including the filtering device.